Dominant allele

In the syllabus and the exam

A dominant allele is defined by what it does in a heterozygous genotype: it is expressed in the phenotype whenever at least one copy is present, masking the effect of a recessive allele on the other chromosome of the pair. Candidates must be able to identify which allele is dominant purely from information given in a question, usually stated directly (for example, 'brown eyes are dominant over blue eyes') rather than assumed from any pattern in the letters used, and must then apply this correctly across a full genetic cross rather than only in the first generation. A frequent error is assuming that the dominant allele is somehow 'stronger' in a biological sense, such as being more common in a population or more likely to be passed on; dominance describes only how the allele is expressed in the phenotype of a heterozygote, and has no direct connection to how frequently that allele appears in a population, which is a separate topic. Candidates should also avoid stating that a dominant allele 'destroys' or 'removes' the recessive allele; the recessive allele is still physically present in the genotype and can still be passed on to offspring, it is simply not expressed in the phenotype of that particular individual because the dominant allele's effect is expressed instead. Candidates should further note that dominance is specific to a particular pair of alleles for one gene, so an allele that is dominant in one cross cannot be assumed to be dominant over a completely different allele of an unrelated gene.

Example

In pea plants, the allele for purple flowers (P) is dominant over the allele for white flowers (p), so a plant with the genotype Pp still has purple flowers because the dominant allele masks the recessive one.

Related terms

Recessive alleleAlleleGenotype

Source:SRC-DSKP-EN